A man reported to be a survivor from the attack east of Damascus on 21 August. Photograph: Stringer/Reuters

Experts who have examined the first video footage of the immediate aftermath of the attack on people on the outskirts of Damascus Wednesday morning believe it shows the most compelling evidence yet consistent with the use of a lethal toxic agent.

Among those interviewed by the Guardian were analysts who had previously raised questions over the authenticity of previous claims or who had highlighted contradictions.

If confirmed, the attack, quickly blamed on the regime of Bashar al-Assad by Syrian opposition sources, would be the worst involving chemical weapons in decades. Estimates of the fatalities ranged between 100 and 1,300 people.

As the White House appealed to the UN for an immediate investigation,it was reported that three separate strikes involving suspected chemical weapons had occurred within a three-mile radius, in the eastern suburbs of Damascus, in the rebel-held areas of Jobar, Zamalka and Ein Tarma.

"The video footage and pictures this time are of a far better quality," said Jean Pascal Zanders, a former analyst with the chemical and biological warfare project at the Stockholm International Peace Research Institute, who has worked in the field of chemical, biological and nuclear proliferation since the mid-1980s and has questioned some previous claims.

Zanders said: "You can clearly see the typical signs of asphyxiation, including a pinkish blueish tinge to the skin colour. There is one image of an adult woman where you can see the tell-tale blackish mark around her mouth, all of which suggests death from asphyxiation.

"What is also different in this footage is that we are seeing the chaos of the first response to what occurred. We are seeing the emergency services being overwhelmed by the innocent victims. It feels very authentic."

Significantly the attack on Wednesday in the Ghouta neighbourhood took place with a UN chemical weapons inspectors' team only about six miles away.

The 20-strong team, led by the Swedish chemical weapons expert Åke Sellström, arrived in Damascus three days ago, though only after months of negotiations between the UN and the Syrian regime over which sites the inspectors could visit.

It seemed unlikely, however, that the inspectors would be able to gain quick access to the site to investigate what had happened.

The footage that emerged from locations apparently hit by a barrage of rockets in the early hours of Wednesday, showed dozens of fatalities, including women and children, as well as a large number of survivors in obvious distress.

An opposition activist and a pharmacist in the town of Arbeen, who identified himself by the pseudonym Abu Ahmad, said he attended to dozens of injured people in a field hospital after the shelling on Zamalka and Ein Tarma.

"Their mouths were foaming, their pupils were constricted, and those who were brought in while still alive could not draw their breaths and died subsequently," he told the Associated Press via Skype. "The skin around their eyes and noses was greyish."

Visible symptoms reported included rolling eyes, foaming at the mouth, and tremors. There was at least one image of a child suffering miosis, the pin-point pupil effect associated with the nerve agent sarin, a powerful neurotoxin reportedly used before in Syria.

Echoing Zanders, Ralph Trapp, a former scientist at the Organisation for the Prohibition of Chemical Weapons, said the footage showed what a chemical weapons attack on a civilian area would look like.

"This is one of the first videos I've seen from Syria where the numbers start to make sense. If you have a gas attack you would expect large numbers of people, children and adults, to be affected, particularly if it's in a built-up area."

While some pro-rebel sources were quick to claim it was sarin that had been used, others were cautious, among them Zanders.

"I remain sceptical that it was a nerve agent like sarin. I would have expected to see more convulsions," he said. "The other thing that seems inconsistent with sarin is that, given the footage of first responders treating victims without proper protective equipment, you would expect to see considerable secondary casualties from contamination – which does not appear to be evident."

Trapp too was cautious. "It is possible a gas was involved, but the images I've seen were not clear enough to see other symptoms beyond difficulty in breathing and suffocation. It certainly looks like some sort of poisoning."

Gwyn Winfield, editor of CBRNe World, a journal serving those tasked with responding to chemical, biological and nuclear events, was also dubious about the use of weapons-grade sarin given the apparent lack of contamination of first responders.

"You would expect to see first responders going down. But if the numbers we are hearing are correct, and a very large number of people have died, it is likely this was not a riot control agent [as has been suspected in some previous events] but something more toxic, perhaps some kind of cocktail containing an organophosphate agent. It doesn't look like phosgene or like hydrogen cyanide."

The Obama administration has stated that the usage or movement of a "whole bunch" of chemical weapons in Syria would cross a "red line" that would trigger fresh US intervention.

One chemical weapons inspector who is not in Syria told the Guardian he was doubtful the UN inspection team would gain quick access.

"I very much doubt the team will be given access to the site. The whole inspection has been delayed for weeks and months already over the formalities of visiting each site."

Many of symptoms seen on Wednesday could be caused by other substances, and chemical weapons inspectors will need to rule these out. Organophosphate pesticides can have similar effects to sarin, and several, including tetraethyl pyrophosphate (TEPP) and parathion, have caused deaths.

Missiles can strike chemical stores, releasing poisonous gases such as chlorine, which is used to sterilise water. Shells that carry sarin can also carry fuel-air explosives, which can cause suffocation; the munition produces a huge cloud of fuel that is ignited to produce a blast and suck huge amounts of oxygen out of the air.

Even if allowed access UN inspectors would have a short while to gather concrete evidence from Ghouta.

For its part, Syria's closest ally, Russia, claimed the attack on Wednesday was a deliberately staged provocation by rebel forces designed to trigger international intervention.

Alexander Lukashevich, a foreign ministry spokesman, said sources in Syria claimed a homemade rocket carrying unidentified chemical substances was launched from an area controlled by the opposition.

What could have been used in Ghouta?

Sarin

A colourless, odourless liquid that kills mostly through inhalation. Like other nerve agents, sarin works by blocking the "off switches" of the nervous system, causing nerves to fire constantly. It causes drooling, sweating and pinpoint pupils and usually kills through suffocation. Contaminated clothes can release sarin for half an hour after exposure, contaminating first-aiders.

Tabun

The first nerve agent to be developed, and the easiest to make, as the chemicals needed can be found on the open market. Tabun has a slightly fruity odour, but the smell may be too weak for people to notice. It will stay on surfaces for longer than sarin.

Soman

The fastest-killing nerve agent, soman works in the same way as other organophosphate nerve agents. It has a slight fruity or camphor odour.

VX

A more potent nerve agent than sarin, VX is believed to be in the Syrian stockpile of chemical weapons. It is an oily amber liquid and, like sarin, has no smell or taste. It works in a similar way to sarin and causes the same symptoms.

Phosgene

An industrial chemical used to make plastics and pesticides. Phosgene was responsible for more deaths than any other chemical in the first world war. High exposures can cause fluid on the lungs, shortness of breath and heart failure.

Insecticides

Nerve agents are the most potent forms of organophosphates, but milder forms are used as insecticides. These can still kill in large quantities. Insecticides developed for farm use such as tetraethyl pyrophosphate (TEPP) and parathion have caused numerous deaths in the past.